1. Field of the Invention
This invention relates to electrical circuits for producing output signals according to a logarithmic function. More particularly, this invention relates to improved circuitry for developing a temperature-independent logarithmic output signal.
2. Description of the Prior Art
Various kinds of analog logarithmic circuits have been used industrially for many years. These have included log-amps (having only one variable input signal) and log-ratio circuits (having two variable input signals). Generally, the logarithmic function is established by a pair of opposed P-N junctions carrying respective currents I.sub.1, I.sub.2, with the differential voltage kT/q (ln I.sub.1 /I.sub.2) being used as the basic output signal. Since the output signal is proportional to absolute temperature, it is evident that some form of temperature compensation must be provided for any such circuitry which is required to function accurately at varying temperatures.
There has been a problem in providing temperature-compensated logarithmic circuits which are suitable for fabrication in monolithic form, i.e. integrated-circuit chips. Commonly, in prior circuits a resistor having a high temperature-coefficient (TC) is used to effect the required temperature compensation. However, providing such a resistor is difficult to do monolithically. As a consequence, an external high-TC resistor generally is employed. This is not satisfactory because the product must then be manufactured in module format rather than as a totally monolithic implementation.